While it is correct that there is very few hard and fast rules, there is in fact acceptable etiquette. Not every one who overnights at Wally adheres to them but they do exist. They are like the "unwritten rules" that govern many things in a polite society. If you do nothing else, please go in and introduce yourself to the manager on duty. It also helps if you happen to have a recent purchase in your hands. If they don't allow overnight parking, s/he will let you know. Likewise, if there is a specific area they would like you to park, or not park, they will let you know. Using this strategy, I've on occasion been given go ahead to stay, even at Wallys that were posted No Overnight Parking. Not often, but it does happen. The alternative is to take your chances with the O'dark hundred knock on the door.
One last tip, since Walmart's corporate policy is to be welcoming to their RV customers whenever possible, many lots will have their more distant parking slots outlined with white paint, while the majority of the spots will be yellow. In theory these are the spots where RVs or employees are supposed to park, leaving the closer spots for other customers. Just a little tip. Take it or leave it as you choose.
We all use our RVs a little differently. That freedom is big part of RVing.